This invention generally relates to containers and more particularly to two or three piece containers made of sheet metal material such as steel or aluminum.
The general concept of an outwardly flexible bottom wall in a metal container has been known at least since U.S. Pat. No. 2,894,844. Such a structure enables the use of less metal in the container, thereby reducing the cost of material. Various other configurations of container bottom end wall structures have previously been suggested as illustrated by U.S. Pat. Nos. 6,391; 79,692; 2,541,065; 2,847,144; 2,929,525; 3,043,461; 3,259,296; 3,430,805; 3,598,270; 3,690,507; and 3,871,541. In addition, a metallic container having a spherical dimple in an outwardly flexible bottom wall providing a continuous circular support rim in an outwardly flexed position has been disclosed in U.S. Pat. No. 3,904,069.
The primary purpose of the present invention is to provide a can-type sheet metal container made of relatively thin sheet stock and having a relatively thin side wall and a relatively thin bottom wall so as to reduce material costs. In a container made of such thin material the bottom wall will be axially outwardly flexible when filled with goods under pressure such as beer, other carbonated beverages, or other consumer goods canned as for example in sanitary cans. In order to enable such containers to be supported in a vertically upright position, new and improved integral support means are provided in the bottom wall to be effective in the outwardly flexed position to provide very stable support for the container, while being capable of manufacture and filling in high speed production lines. Although various integral support structures have been previously proposed, the results have not always been satisfactory from the standpoint of displacement of internal volume inside the can, reliable stability, internal coatability and of ease of manufacture at high production speeds. In most presently utilized can structures, the bottom wall is relatively thick and has either a flat ribbed bottom surface providing support for the can or an axially inwardly extending concave configuration locating the center portions of the bottom wall axially inwardly of a bottom rim portion of the can. In an outwardly convex configuration portions of the bottom wall unpredictably extend varying axial distances beyond the bottom rim portion and because of the convex curvature produce an unstable condition when the can is placed on a flat support surface. In order to provide stability for such cans, it has been previously proposed to provide integral support means in the bottom wall portion effective in the outwardly convex configuration. However, it has been determined that in connection with the present invention there are inherent problems in the manufacture of such cans including obtaining uniform dimensional characteristics and uniform residual stresses in the bottom end wall so that each bottom end wall of mass produced cans will have the same or similar characteristics without resorting to exceptional manufacturing means for achieving metal control such as double acting domer cylinders, etc.
In the present invention, the bottom wall is manufactured and constructed in a manner to obtain uniform axial displacement during outward flexing while also providing for increased uniform stability of such cans in the outwardly flexed position. The central portion of the bottom wall is first formed into an axially inwardly extending concave portion of compound curvature within a central bottom wall area of polygonal configuration, an equilateral triangular area being presently preferred, and equilaterally spaced linear support areas are provided about the area of compound curvature in the portions of the bottom wall furthest axially outwardly deflected in the outwardly flexed position. In this manner, at least three support areas providing essentially point or short length line contact with a flat support surface are located between the bottom rim of the can and the central concave portion of the bottom wall so that the can is supported by relatively widely spaced relatively narrow width and short length support areas providing stability for the can. In the manufacture of cans having the aforedescribed construction, the central concave portion is formed in the bottom wall with a generally hemispherical doming die, and an ironing punch having a prismatic cavity of polygonal, preferably equilateral triangular, base section coaxially aligned with and surrounding the dome portion of the doming die when the ironing punch and doming die are in the closed position. Due to varying metal stress the inwardly concave central portion of the bottom wall thus formed is of compound curvature with a relatively short length radius extending from each of the support areas toward the central axis and a relatively long arcuate surface of relatively long length radius extending opposite each of the support areas from the central axis toward the apex portions of the polygon. The arrangement is such that when the bottom wall is subsequently flexed outwardly under pressure of the contents of the can, the support areas are located furthest axially outwardly and the adjacent areas are axially inwardly displaced relative thereto so that only the support areas contact a flat support surface for the can.